WO 98/01083 discloses a toothbrush with a brush-head carrier and a handle. The brush-head carrier is connected to, and projects out from, the handle. In the interior of the brush-head carrier is a drive shaft for driving the brush head. The drive shaft is mounted in a way that allows it to pivot in a rocking motion relative to the handle. The end of the brush-head carrier that is enclosed by the handle is biased against an eccentric by a spring. The toothbrush's drive motor drives the eccentric. Thus, the brush-head carrier follows the driven movement of the eccentric, making an oscillating rocking motion. As a result, the brush head moves in a reciprocating or picking fashion, thereby better removing impurities on the teeth, e.g., plaque.
Such a picking motion by the brush head, however, can irritate or even damage the gums. This irritation or damage is especially likely to occur when the brush head exerts excessive contact pressure against the teeth. Consequently, it has been proposed to equip the toothbrush with the ability to monitor contact pressure. A pressure sensor senses the contact pressure and indicates to the toothbrush operator whether cleaning is being carried out with the correct contact pressure. For example, WO 92/13499 provides a toothbrush with a drive shaft that can be pivoted out of a rest position, counter to a restoring force, transversely to the center longitudinal axis of the toothbrush. When the appropriate contact pressure has been reached, the toothbrush informs the toothbrush operator by means of acoustic or optical signals.
In other electric toothbrushes, such as those in DE 44 26 446 A1 and EP 0 158 870 B1, a pressure sensor measures the reaction force to the contact pressure. The pressure sensor is located on the brush-head carrier and/or the drive shaft.
The current solutions for sensing contact pressure employ relatively expensive and sensitive pressure sensors. Additionally, they use expensive signal-processing means. Sensing contact pressure in this way is especially difficult in the case of toothbrushes that execute a reciprocating or picking movement. A more simple solution is to sense contact pressure by using contacts that detect a cleaning-force threshold. However, such contacts only detect one cleaning-force threshold. In other words, they are not capable of notifying the operator both when the cleaning force has dropped below a lower limit and when it has exceeded an upper limit.